JavaScript Promises

In a JavaScript context, you often need to deal with asynchronicity and this often involves promises. However, when working with promises in ClojureScript, getting at the value of a promise can be a bit tedious. I often see the following pattern for println debugging.

(defn async-fn []
  (.resolve js/Promise :hello))

(.then (async-fn) println)

I'm not a huge fan of this pattern, and worse yet, if you forget to wrap the value, you get #object [Promise [object Promise]] which isn't very helpful.

Thankfully, in ClojureScript you have tap> which can help you with this issue.

With portal, you would typically do something like:

(require '[portal.api :as p])
(add-tap #'p/submit)

(tap> (async-fn)) ;; #object [Promise [object Promise]]

But this has the same issue as before when you printed the promise value. To improve tap> usage around promises, you simply need to provide a specialized tap target, like the following:

(require '[portal.api :as p])

(defn async-submit [value]
  (if-not (instance? js/Promise value)
    (p/submit value)
    (-> value
        (.then p/submit)
        (.catch p/submit))))

(add-tap #'async-submit)

(tap> (async-fn)) ;; :hello